1. Field of the Invention
The present invention relates to a method and apparatus for determining a measured direction relative to an external magnetic field and for indicating this measured direction in digital or analog form for display or utilization in operating an error function.
The external magnetic field of most common interest, which forms the basis of much terrestrial navigation for ships, airplanes, automobiles, or other means of transportation, is the earth's magnetic field. Since this field is relatively constant, it is a convenient reference against which other variable directions are measured.
It may also be of interest to determine a measured direction relative to some external magnetic field other than the earth's or to detect changes in the relative external magnetic field caused by external ferromagnetic objects or apparatus motion. The method and apparatus of the present invention can be used in any such application.
2. Description of the Prior Art
Devices which determine a measured direction relative to the earth's magnetic field are usually called compasses. Though various forms of mechanical compasses have been known for centuries and improvements to them have continuously been made, the basic principle of their operation remains unchanged. In particular, mechanical compasses generally include some form of an elongate magnetic bar rotatably mounted so that it can align itself with the earth's field. A compass card or scale is mounted relative to the bar which an operator can then read to determine the measured direction.
Mechanical compasses are usually suspended in a gimbal to maintain their level attitude when their support is tipped, and dampened to lessen the effect of external vibrations which they might experience. However, certain gimbals are complex, awkward, and expensive. Mechanical dampening methods and devices are not well suited for use in compasses employed in some applications.
Electronic compasses have been developed to overcome certain difficulties associated with mechanical compasses. For example, U.S. Pat. No. 3,825,823 -- Rovner discloses an apparatus for detecting a magnetic field relative to a marker direction, that operates without moving parts, incorporating a flux gate magnetometer having a plurality of sensing coil arrays each aligned with a given direction. The flux gate is driven by an R. F. generator. A scanner sequentially connects each array to a detector to determine which sensing coil system of two coil arrays is perpendicular to the external field. The resolving power or ability of this apparatus to distinguish between small angular differences in direction is determined by the number of sensing coil arrays which are coupled to the flux gate, and is, in practicality, limited by the number the flux gate can physically accommodate. Furthermore, the electronic circuitry for scanning each sensing coil system is relatively complex and its complexity increases when large numbers of sensing coils are employed to improve the angular resolution.
U.S. Pat. No. 2,922,227 -- Turner et al. discloses a similar device which includes a flux gate comprising a ring core, a toroidal coil wound about the ring, and a pair of sensing coils coupled at diametrically opposed positions to the ring. The ring is rotationally driven in response to the magnetic field sensed by the flux gate to maintain a particular attitude with respect to the external field. The position of the ring is then used to indicate the position of the external field.
U.S. Pat. No. 2,924,886 -- Cullen discloses a transmitting magnetic compass that includes a semiconductor sensing element that is arranged to rotate to align itself in a particular attitude with the external field. The angular position of the semiconductor is transmitted to a receiver to be read by an operator as the direction of the external field.
Both the Cullen and Turner et al. patents disclose means for tracking the external field which are then sensed to determine the external field's direction. In addition, both apparatus disclosed employ moving parts and, therefore, are characterized by certain disadvantages of conventional mechanical compasses.
Other devices which employ flux gate magnetometers are disclosed in U.S. Pat. Nos. 2,426,470 -- Sinks and 2,342,637 -- Bechberger. Other compass constructions are disclosed in U.S. Pat. Nos. 3,833,901 -- Fowler; 2,886,989 -- Karlson; 2,755,562 -- Hurlburt; 2,611,191 -- Noxon et al.; 2,502,786 -- Hayslett; and 2,454,184 -- Kliever. These latter patents have been reviewed and are considered more remote from the present invention then the disclosures reviewed in detail. They are cited in order to be complete and in case the reader deems them of interest.